The ability to excite a sample within a sample volume in a spatially selective manner is a fundamental technique for MRI imaging, and most MRI equipment is able to do this. For example, the most common spatially selective excitation involves selectively exciting a slice within the sample while leaving the rest of the sample undisturbed. In a multi-slice mode, a series of (usually parallel) slices are excited in quick succession. Other loci of spatial selection are used for different reasons, including those that correspond to axes of Cartesian, polar, cylindrical or other coordinatizations of the sample volume, and other loci that involve discontinuous selections, such as in a 2D checkerboard pattern.
In conventional MRI apparatus, slice selection is conventionally implemented using a shaped radio-frequency (RF) pulse applied simultaneously with a gradient in magnetic gradient strength, which is equivalent to a gradient field in resonant frequency.
Applicant's U.S. Pat. No. 7,141,973 teaches a method of effecting magnetic resonance experiments where the RF transmission coil is used to spatially encode the signal as a replacement for or a supplement to applying steps with magnetic field gradients. No method for spatially selective excitation within the sample volume is taught, and none would be expected for a NMR apparatus having the limited resolution demonstrated in '973.
It would be desirable to improve on the system for magnetic resonance imaging without use of gradient coils, to provide for spatially selective excitation. Preferably a high degree of specificity of spatial selectivity is preferred.